Evolution of Langmuir circulation during a storm

Wind stress, waves, stratification, velocity profiles, and surface fields of radial velocity and acoustic backscatter intensity were measured along a drift track 50 to 150 km off Point Arguello, California. On March 8, 1995, the wind increased from calm to 12 m/s from the SE, opposing swell from the NW. It increased to 15 m/s at noon UTC on March 9, remained steady over the next 12 hours, briefly dropped and veered by 60°, then returned. A mixed layer deepened quickly to 25 m, then held roughly steady through the next 2 days, in spite of gusty winds continuing at 15–25 m/s. A phased-array Doppler sonar system took measurements covering 250 m by 150 m of the surface, with 5 m by 10 m spatial resolution. Averages over 6 min removed surface waves, permitting continuous assessment of strength, orientation, spacing, and degree of organization of features associated with surface motion (e.g., Langmuir circulation), even when conditions were too rough for visual assessment. Several results stand out: (1) As found previously, most wind mixing arises from inertial shear across the thermocline. (2) Consistent with wind/wave forcing of Langmuir circulation, Plueddemann et al. [1996] suggest that surface velocity variance scales like (u*Us), where u* is friction velocity and Us is the surface Stokes' drift; however, the measurements here scale with (Us)1 alone, once Langmuir circulation is established. (3) The velocity variance is weaker here than expected, given the magnitudes of wind and waves, leading to a smaller estimated mixing effect. (4) Large vacillations in LC strength are seen just before the brief veering of the wind; it is suggested that bubble buoyancy could play a dynamic role. (5) Mean orientation and spacing can differ for intensity versus radial velocity features.